Solar thermal energy systems represent a technology for capturing renewable radiant energy from the sun and converting that energy into thermal energy that can be used to generate electricity. Concentrating solar power (CSP) is a technology that offers electric utility scale power production. CSP systems include collectors such as mirrors or reflectors (sometimes referred to as heliostats or concentrators) that are arrayed in a solar collector field (“solar field” or “SF”) which capture and in turn concentrate sunlight onto a thermal receiver. The thermal receiver contains a heat transfer fluid such as oil or molten salt (typically a mixture of 60% sodium nitrate and 40% potassium nitrate) that is heated to a temperature sufficient to produce steam via a combination of conventional fluid-to-steam heat exchangers. The steam is used to drive a conventional steam turbine-generator set (“power block” or “PB”) which produces electricity that may be sold to a power grid operated by a an electric power distribution company or utility for delivery to its customers over a conventional power transmission network. Some present CSP system designs include parabolic trough systems, parabolic dish systems, and power tower systems that employ a plurality of reflectors which focus the solar energy onto a thermal receiver positioned atop a centrally-located tower.
Thermal energy storage (TES) is an integral part of CSP systems for capturing and storing as much solar thermal energy as possible when available to compensate for periods of time when sunlight is not available due to either weather conditions or time of day. TES basically employs an insulated hot storage tank and a pumping and piping system with suitable flow control valves which may temporarily store the heated heat transfer fluid or medium until needed to produce steam for generating electricity via the power block. In some systems, a combination of oil and molten salt may be used as the heat transfer fluids coupled with a combination of oil-to-salt and/or oil or salt-to-steam heat exchangers. In other systems, a single heat transfer fluid may be used. The heat exchangers are not 100% efficient; therefore, there will be thermal energy losses incurred when heat is exchanged. A typical heat exchanger efficiency without limitation is about 92% as an illustration. Accordingly, the net amount of thermal energy that may be either stored in TES or transferred to the power block will be less than the thermal energy produced by solar collector field.
Two types of TES systems are generally employed—direct storage and indirect storage TES. In direct TES, as shown in FIG. 1, a serial pumping and piping arrangement are used in the CSP system 15 between the solar collector field 10, thermal energy storage 12 which may comprise one or more conventional insulated storage tanks, and power block 14. Both the solar collector field 10 and thermal energy storage 12 may have one or more associated pumps that cause the heat transfer fluid or medium to flow in the desired direction through flow conduits 18A-B (and 18C shown in FIG. 2 discussed below). Whenever the solar collector field 10 generates thermal energy, it is sent to the TES. If the TES is full, the energy is dumped or wasted. When there is a demand to produce electric power, thermal energy is drawn from the TES storage tank to produce steam and drive the turbine-generator set of the power block. The embodiment shown employing dual heat transfer fluids requires both an oil-to-salt heat exchangers 11 and a salt-to-steam heat exchanger 13.
For indirect TES, as shown in FIG. 2, parallel pumping and piping arrangements are used in the CSP system 16 between the solar collector field 10, thermal storage 12, and power block 14. Therefore, thermal energy may be routed whenever generated by solar collector field directly to the power block 14 (an via oil-to-steam heat exchanger 17) and/or to the TES (via oil-to-salt heat exchanger 11) depending on whether there is a demand to produce electric power and/or the amount of thermal energy needed to produce sufficient steam to drive the turbine-generator set of the power block. The power block may draw thermal energy from both TES (via salt-to-steam heat exchanger 13) and directly from the solar collector field if needed.
Both direct and indirect TES have advantages and disadvantages. The overall efficiency of indirect TES is higher than direct storage (generally about 8% more in some instances) because two heat exchanges are not always involved in the thermal energy flow between the solar collector field and power block as shown in FIG. 2. The efficiency of direct TES is therefore inherently lower because two heat exchangers are always used in the system as shown in FIG. 1. However, direct TES has a simpler control system and is generally easier to optimize and schedule its operation. Optimization is more difficult with indirect TES because of its flexible operation since scheduling when to dispatch of thermal energy from the heated TES tank to the power block depends on the hours selected for electricity generation. In direct TES, by contrast, thermal energy must always pass through and be drawn from the heated storage tank whenever there is a demand to produce steam for generating electricity. Accordingly, indirect TES requires a more complex control system than direct TES.
Accordingly, an apparatus and method for better optimizing operation of CSP systems with indirect TES is desired to take advantage of its higher efficiencies in contrast to direct TES.